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Components of deferred tax assets and liabilities (Details 3) (USD $) | Dec. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Valuation Allowance, Amount | $ 0 | $ 0
Deferred tax assets: |  |
Regulatory matters | 125,607,000 | 121,781,000
Accrued pension costs | 74,320,000 | 85,037,000
Alternative minimum tax credit carryforward | 33,304,000 | 0
Compensation-related | 31,550,000 | 23,441,000
Asset retirement obligations | 29,578,000 | 26,748,000
Legal and environmental contingencies | 10,710,000 | 8,046,000
Other | 45,101,000 | 39,792,000
Total deferred tax assets | 350,170,000 | 304,845,000
Deferred tax liabilities: |  |
Depreciation and basis differences on property, plant and equipment | 813,597,000 | 755,392,000
Basis differences on oil and natural gas producing properties | 266,168,000 | 167,113,000
Regulatory matters | 64,914,000 | 82,077,000
Intangible asset amortization | 13,579,000 | 14,078,000
Other | 26,170,000 | 18,441,000
Total deferred tax liabilities | 1,184,428,000 | 1,037,101,000
Net deferred income tax liability | $ (834,258,000) | $ (732,256,000)

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